Herpes simplex virus can undergo either a productive infection, where all the viral genes are expressed culminating in the production of progeny virus and cell death, or it can enter a latent state, which is characterized by the relative lack of viral gene expression, genome persistence, and cell survival. The latent state typically occurs only in neurons, and may involve the attenuation of immediate early (IE) gene expression, and thus the lack of later viral gene expression. The IE protein ICP0 has been shown to facilitate the transition from the latent to the lytic state, thus leading to reaction episodes that are typical of .-herpesviruses. We have been investigating the behavior of viruses that do not express any of the five IE proteins upon infection of any cell. The genomes are transcriptionally quiescent, persist in cells, and cytopathic effects are not seen. They can be rendered transcriptionally active by supplying ICP0 in trans. Thus this system has some of the key features associated with HSV latency, but is also more amenable to biochemical and molecular study. Over the past funding period we have found that;quiescent genomes are tightly packed in chromatin with hypoacetylated histones in a state resembling heterochromatin, persisting genomes are circular, and an antiviral response is generated following infection with viruses that don't express IE genes. Importantly, the addition of ICP0 inhibits and/or reverses all of these processes. In the present application we propose to further study the effects of ICP0 on chromatin and circularization of the HSV genome. Four specific aims are proposed: (i.) further investigate the state of persisting viral genomes with respect to overall structure, type of chromatin, and the modification state of specific residues of specific histones residing on persisting genomes, (ii.) determine how ICP0 affects the modification state of specific histone residues to gain an understanding of the cellular chromatin remodeling pathways affected by ICP0, and hence potentially involved in reactivation, (iii.) determine the cellular DNA repair pathways and specific proteins that are affected by ICP0, thus inhibiting the circularization of the HSV genome, and (iv.) determine the state of persisting genomes in neuronal cells.